The interest within the scientific community in high-power, subpicosecond, terahertz (10.sup.12 Hz) bandwidth radiation is growing rapidly because of the wide range of applications in which such radiation may be utilized. Among these applications are communications, electronic material characterization, and the development of high-speed optoelectronic devices.
Currently, there are two basic approaches for generating terahertz (THz) beams: (1) using photoconductors and (2) using nonlinear optical frequency conversion techniques. In the photoconductive approach, electrically biased high-speed photoconductors are used as transient current sources for radiating antennas, including elementary Hertzian dipoles, resonant dipoles, tapered antennas, transmission lines, and large-aperture photoconducting antennas. In the nonlinear optical frequency conversion approach, second-order or higher-order nonlinear optical effects in unbiased materials are used. By far, the most important of these nonlinear optics approaches is the optical rectification technique. The optical rectification approach is simpler than the photoconductive approach since no electrical bias is required.
Electro-optic crystals are crystals having an index of refraction that changes in proportion to an applied dc or low-frequency electric field. The magnitude of the effect is proportional to the magnitude of the crystal's electro-optic coefficients.
Prior research has examined several nonlinear materials as terahertz optical rectification media. The materials examined have included organic salts, such as dimethyl amino 4-N-methylstilbazolium tosylate (DAST), traditional electro-optic materials, such as LiTaO.sub.3 and LiNbO.sub.3, and semiconductors, such as GaAs. To date, the most efficient terahertz medium has been found to be DAST.
Other commonly used electro-optic materials include NH.sub.4 H.sub.2 PO.sub.4 (Ammonium dihydrogen phosphate or ADP), KH.sub.2 PO.sub.4 (Potassium dihydrogen phosphate or KDP), and CdTe (Cadmium telluride). Still more materials which are structurally similar to ADP include Deuterated KDP or KD*P, rubidium dihydrogen phosphate or RDP, ammonium dihydrogen arsenate or ADA, potassium dihydrogen arsenate or KDA, rubidium dihydrogen arsenate or RDA, cesium dihydrogen arsenate or CDA, and deuterated CDA. Materials which are structurally similar to LiNbO.sub.3 include barium titanate or BaTiO.sub.3, and lithium iodate or LiIO.sub.3. Materials structurally similar to CdTe include zinc sulfide or ZnS, zinc telluride or ZnTe, and zinc setenide or ZnSe. Other materials which might also serve as electro-optic material include quartz, CuCl, InAs, InP, and GaP.
In this method, the nonlinear material is illuminated with ultrashort laser pulses, causing a time-dependent polarization to be created in the material by way of the electro-optic effect. This induced polarization is proportional to the intensity of the excitation pulse, and produces radiation of electromagnetic waves having a terahertz bandwidth. With a suitable electro-optic material, the amplitude of the resulting terahertz field is controlled by the intensity of the optical excitation beam. In turn, this intensity is a result of the pulsewidth, energy, and spot size of the beam.
The only problem with this method is that at high optical intensities, the efficiency of the rectification mechanism may decrease due to competing nonlinear effects. The incident optical intensity at which these competing mechanisms will occur is material dependent. Testing has indicated that, with DAST, for pulsewidths on the order of 215 fs, the saturation in rectification efficiency became pronounced at fluences greater than 20 mJ/cm.sup.2. This saturation is directly related to the size of the DAST crystal for a given excitation energy. Therefore, in order to generate high energy terahertz beams, it is necessary to use large aperture crystal emitters. However, the problem with creating large aperture electro-optic crystals is that large, optical quality electro-optic crystals cannot be easily or efficiently grown. The typical optical quality DAST crystal can be grown as platelets up to approximately 5 mm by 5 mm by 1 mm in size.
It is therefore an object of the present invention to provide a large aperture electro-optic emitter for use in generating terahertz radiation.
It is another object of the present invention to provide large aperture electro-optic emitters made as a mosaic of individual electro-optic crystals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.